1. Field of the Invention
The present invention relates to a fuel supply system for an internal combustion engine.
2. Description of the Related Art
There have hitherto been known a variety of fuel supply systems for internal combustion engines, such as a technique of increasing and decreasing the number of operations of fuel pumps (i.e., the number of operating fuel pumps) according to the operating conditions of an internal combustion engine (for example, see a first patent document: Japanese patent application laid-open No. 03-9067 (pages 2, 3, and FIG. 2)), a technique of operating only one of fuel pumps at the time of engine starting (for example, see a second patent document: Japanese patent application laid-open No. 03-74564 (pages 3, 4, and FIGS. 3 and 4)), a technique of operating only one of fuel pumps at the time when the amount of fuel to be injected is limited (for example, see a third patent document: Japanese patent application laid-open No. 05-157013 (pages 2, 3, and FIG. 2)), a technique of using two fuel pumps for two common-rails, respectively (for example, see a fourth patent document: Japanese patent application laid-open No. 11-44276 (pages 3-6, and FIG. 1)), and a technique of using a plurality of fuel pumps with a different one thereof being operated every time an engine is started (for example, see a fifth patent document: Japanese patent application laid-open No. 10-259769 (pages 2-5, and FIG. 2)).
However, in cases where an internal combustion engine is equipped with a plurality of fuel pumps, the pressure of fuel supplied by these fuel pumps is caused to vary when the number of operations of the fuel pumps is changed. In addition, parts of fuel discharged from the respective fuel pumps might interfere with one another, so it would become difficult to suppress pulsations of the fuel pressure. If the fuel pressure is varied in this manner, it becomes difficult to perform stable fuel injection.